commit 829c25f93e856f56889e91584da59084f13ac3f1
Author: Pierre Rineau <pierre.rineau@makina-corpus.com>
Date:   Sun Feb 26 14:10:33 2017 +0100

    Issue #1545964 by Pol, pounard, Rene Bakx, Kars-T, SebCorbin: Do not copy over the owner and engine of a theme if the child theme uses a different engine than the base theme

diff --git a/includes/module.inc b/includes/module.inc
index 2e251080b7..10d1b1c49f 100644
--- a/includes/module.inc
+++ b/includes/module.inc
@@ -193,17 +193,12 @@ function system_list($type) {
           foreach (array_keys($lists['theme'][$key]->base_themes) as $base_theme) {
             $lists['theme'][$base_theme]->sub_themes[$key] = $lists['theme'][$key]->info['name'];
           }
-          // Add the base theme's theme engine info.
-          $lists['theme'][$key]->info['engine'] = isset($lists['theme'][$base_key]->info['engine']) ? $lists['theme'][$base_key]->info['engine'] : 'theme';
-        }
-        else {
-          // A plain theme is its own engine.
-          $base_key = $key;
-          if (!isset($lists['theme'][$key]->info['engine'])) {
-            $lists['theme'][$key]->info['engine'] = 'theme';
-          }
         }
         // Set the theme engine prefix.
+        // During 6.x to 7.x upgrade, 6.x themes may not have an 'engine' key.
+        if (empty($lists['theme'][$key]->info['engine'])) {
+          $lists['theme'][$key]->info['engine'] = 'phptemplate';
+        }
         $lists['theme'][$key]->prefix = ($lists['theme'][$key]->info['engine'] == 'theme') ? $base_key : $lists['theme'][$key]->info['engine'];
       }
       cache_set('system_list', $lists, 'cache_bootstrap');
diff --git a/includes/theme.inc b/includes/theme.inc
index 9b606e9fb1..43a75faeb4 100644
--- a/includes/theme.inc
+++ b/includes/theme.inc
@@ -139,7 +139,7 @@ function drupal_theme_initialize() {
  *   The callback to invoke to set the theme registry.
  */
 function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callback = '_theme_load_registry') {
-  global $theme_info, $base_theme_info, $theme_engine, $theme_path;
+  global $theme_info, $base_theme_info, $theme_path;
   $theme_info = $theme;
   $base_theme_info = $base_theme;
 
@@ -201,37 +201,24 @@ function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callb
     drupal_add_js($script, array('group' => JS_THEME, 'every_page' => TRUE));
   }
 
-  $theme_engine = NULL;
-
-  // Initialize the theme.
-  if (isset($theme->engine)) {
-    // Include the engine.
-    include_once DRUPAL_ROOT . '/' . $theme->owner;
-
-    $theme_engine = $theme->engine;
-    if (function_exists($theme_engine . '_init')) {
-      foreach ($base_theme as $base) {
-        call_user_func($theme_engine . '_init', $base);
-      }
-      call_user_func($theme_engine . '_init', $theme);
-    }
-  }
-  else {
-    // include non-engine theme files
-    foreach ($base_theme as $base) {
-      // Include the theme file or the engine.
-      if (!empty($base->owner)) {
-        include_once DRUPAL_ROOT . '/' . $base->owner;
-      }
+  // Initialize all themes and engines.
+  // Make sure of loading the current theme first so preprocess functions are
+  // loaded properly.
+  foreach (array($theme) + list_themes() as $current) {
+    // Do not attempt to load en empty owner, this could happen with malformed
+    // theme info, or missing parent theme engine; during runtime initialization
+    // we should not let Drupal emit warnings on a live site.
+    if ($current->owner) {
+      include_once DRUPAL_ROOT . '/' . $current->owner;
     }
-    // and our theme gets one too.
-    if (!empty($theme->owner)) {
-      include_once DRUPAL_ROOT . '/' . $theme->owner;
+
+    if (function_exists($current->engine . '_init')) {
+      call_user_func($current->engine . '_init', $current);
     }
   }
 
   if (isset($registry_callback)) {
-    _theme_registry_callback($registry_callback, array($theme, $base_theme, $theme_engine));
+    _theme_registry_callback($registry_callback, array($theme, $base_theme));
   }
 }
 
@@ -304,8 +291,6 @@ function _theme_registry_callback($callback = NULL, array $arguments = array())
  * @param $base_theme
  *   An array of loaded $theme objects representing the ancestor themes in
  *   oldest first order.
- * @param $theme_engine
- *   The name of the theme engine.
  * @param $complete
  *   Whether to load the complete theme registry or an instance of the
  *   ThemeRegistry class.
@@ -313,7 +298,7 @@ function _theme_registry_callback($callback = NULL, array $arguments = array())
  * @return
  *   The theme registry array, or an instance of the ThemeRegistry class.
  */
-function _theme_load_registry($theme, $base_theme = NULL, $theme_engine = NULL, $complete = TRUE) {
+function _theme_load_registry($theme, $base_theme = NULL, $complete = TRUE) {
   if ($complete) {
     // Check the theme registry cache; if it exists, use it.
     $cached = cache_get("theme_registry:$theme->name");
@@ -322,7 +307,7 @@ function _theme_load_registry($theme, $base_theme = NULL, $theme_engine = NULL,
     }
     else {
       // If not, build one and cache it.
-      $registry = _theme_build_registry($theme, $base_theme, $theme_engine);
+      $registry = _theme_build_registry($theme, $base_theme);
       // Only persist this registry if all modules are loaded. This assures a
       // complete set of theme hooks.
       if (module_load_all(NULL)) {
@@ -672,10 +657,8 @@ function _theme_process_registry(&$cache, $name, $type, $theme, $path) {
  * @param $base_theme
  *   An array of loaded $theme objects representing the ancestor themes in
  *   oldest first order.
- * @param $theme_engine
- *   The name of the theme engine.
  */
-function _theme_build_registry($theme, $base_theme, $theme_engine) {
+function _theme_build_registry($theme, $base_theme) {
   $cache = array();
   // First, process the theme hooks advertised by modules. This will
   // serve as the basic registry. Since the list of enabled modules is the same
@@ -698,15 +681,15 @@ function _theme_build_registry($theme, $base_theme, $theme_engine) {
   foreach ($base_theme as $base) {
     // If the base theme uses a theme engine, process its hooks.
     $base_path = dirname($base->filename);
-    if ($theme_engine) {
-      _theme_process_registry($cache, $theme_engine, 'base_theme_engine', $base->name, $base_path);
+    if (isset($base->engine)) {
+      _theme_process_registry($cache, $base->engine, 'base_theme_engine', $base->name, $base_path);
     }
     _theme_process_registry($cache, $base->name, 'base_theme', $base->name, $base_path);
   }
 
   // And then the same thing, but for the theme.
-  if ($theme_engine) {
-    _theme_process_registry($cache, $theme_engine, 'theme_engine', $theme->name, dirname($theme->filename));
+  if (isset($theme->engine)) {
+    _theme_process_registry($cache, $theme->engine, 'theme_engine', $theme->name, dirname($theme->filename));
   }
 
   // Finally, hooks provided by the theme itself.
@@ -1167,16 +1150,14 @@ function theme($hook, $variables = array()) {
     $extension = '.tpl.php';
 
     // The theme engine may use a different extension and a different renderer.
-    global $theme_engine;
-    if (isset($theme_engine)) {
-      if ($info['type'] != 'module') {
-        if (function_exists($theme_engine . '_render_template')) {
-          $render_function = $theme_engine . '_render_template';
-        }
-        $extension_function = $theme_engine . '_extension';
-        if (function_exists($extension_function)) {
-          $extension = $extension_function();
-        }
+    if (isset($info['engine'])) {
+      $theme_engine = $info['engine'];
+      if (function_exists($theme_engine . '_render_template')) {
+        $render_function = $theme_engine . '_render_template';
+      }
+      $extension_function = $theme_engine . '_extension';
+      if (function_exists($extension_function)) {
+        $extension = $extension_function();
       }
     }
 
@@ -1309,10 +1290,21 @@ function drupal_find_theme_templates($cache, $extension, $path) {
   // used for filtering. This allows base themes to have sub-themes in its
   // folder hierarchy without affecting the base themes template discovery.
   $theme_paths = array();
-  foreach (list_themes() as $theme_info) {
+  $current_theme = NULL;
+  $current_engine = NULL;
+  foreach (list_themes() as $key => $theme_info) {
     if (!empty($theme_info->base_theme)) {
       $theme_paths[$theme_info->base_theme][$theme_info->name] = dirname($theme_info->filename);
     }
+    $theme_path = dirname($theme_info->filename);
+    if ($path === $theme_path) {
+      $current_theme = $current_theme;
+      $current_engine = isset($theme_info->engine) ? $theme_info->engine : NULL;
+    }
+    else if (!$current_theme && 0 === strpos($theme_path, $current_theme)) {
+      $current_theme = $current_theme;
+      $current_engine = isset($theme_info->engine) ? $theme_info->engine : NULL;
+    }
   }
   foreach ($theme_paths as $basetheme => $subthemes) {
     foreach ($subthemes as $subtheme => $subtheme_path) {
@@ -1349,6 +1341,7 @@ function drupal_find_theme_templates($cache, $extension, $path) {
     if (isset($cache[$hook])) {
       $implementations[$hook] = array(
         'template' => $template,
+        'engine' => $current_engine,
         'path' => dirname($file->uri),
       );
     }
@@ -1375,6 +1368,7 @@ function drupal_find_theme_templates($cache, $extension, $path) {
           $arg_name = isset($info['variables']) ? 'variables' : 'render element';
           $implementations[strtr($file, '-', '_')] = array(
             'template' => $file,
+            'engine' => $current_engine,
             'path' => dirname($files[$match]->uri),
             $arg_name => $info[$arg_name],
             'base hook' => $hook,
diff --git a/includes/theme.maintenance.inc b/includes/theme.maintenance.inc
index 6baf219b03..df8d4464d6 100644
--- a/includes/theme.maintenance.inc
+++ b/includes/theme.maintenance.inc
@@ -87,8 +87,8 @@ function _drupal_maintenance_theme() {
 /**
  * Builds the registry when the site needs to bypass any database calls.
  */
-function _theme_load_offline_registry($theme, $base_theme = NULL, $theme_engine = NULL) {
-  return _theme_build_registry($theme, $base_theme, $theme_engine);
+function _theme_load_offline_registry($theme, $base_theme = NULL) {
+  return _theme_build_registry($theme, $base_theme);
 }
 
 /**
diff --git a/modules/simpletest/tests/theme.test b/modules/simpletest/tests/theme.test
index 5f095bd558..d2899a5d42 100644
--- a/modules/simpletest/tests/theme.test
+++ b/modules/simpletest/tests/theme.test
@@ -677,3 +677,122 @@ class ModuleProvidedThemeEngineTestCase extends DrupalWebTestCase {
   }
 
 }
+
+/**
+ * Tests the multi theme engine support.
+ */
+class ThemeEngineNyanCatTestCase extends DrupalWebTestCase {
+  public static function getInfo() {
+    return array(
+      'name' => 'Theme engine test',
+      'description' => 'Tests the theme engine.',
+      'group' => 'Theme',
+    );
+  }
+
+  function setUp() {
+    parent::setUp('theme_test');
+    theme_enable(array('test_theme'));
+  }
+
+  /**
+   * Ensures a theme's template is overridable based on the 'template' filename.
+   */
+  function testTemplateOverride() {
+    variable_set('theme_default', 'test_theme_nyan_cat');
+    variable_set('admin_theme', 'test_theme_nyan_cat');
+
+    $this->drupalGet('theme-test/template-test-engine');
+    $this->assertText('Success: Template overridden with Nyan Cat theme. All of them', 'Template overridden by Nyan Cat file.');
+  }
+
+}
+
+/**
+ * Tests the multi theme engine support with a subtheme.
+ */
+class ThemeEngineNyanCatSubThemeTestCase extends DrupalWebTestCase {
+  public static function getInfo() {
+    return array(
+      'name' => 'Sub theme engine test',
+      'description' => 'Tests the theme engine on a subtheme.',
+      'group' => 'Theme',
+    );
+  }
+
+  function setUp() {
+    parent::setUp('theme_test');
+    theme_enable(array('test_theme', 'test_theme_nyan_cat', 'test_subtheme_nyan_cat'));
+  }
+
+  /**
+   * Ensures a theme's template is overridable based on the 'template' filename.
+   */
+  function testTemplateOverride() {
+    variable_set('theme_default', 'test_subtheme_nyan_cat');
+    variable_set('admin_theme', 'test_subtheme_nyan_cat');
+
+    $this->drupalGet('theme-test/template-test-engine');
+    $this->assertText('Success: Template overridden with Nyan Cat theme. All of them', 'Template overridden by Nyan Cat file.');
+  }
+
+  /**
+   * Ensures that a module providing both templates with different engines
+   * will all be correctly displayed, no matter is the theme, using their
+   * respective engines.
+   *
+   * Tests that phptemplate -> nyan_cat template override works.
+   */
+  function testEngineCohabitationFromDefaultToNyan() {
+    variable_set('theme_default', 'test_theme');
+    variable_set('admin_theme', 'test_theme');
+
+    $this->drupalGet('theme-test/engine-cohabitation');
+    $this->assertText('Inheritance 1.');
+    $this->assertText('Inheritance 2.');
+    $this->assertText('Inheritance 3.');
+    // This one is overriden.
+    $this->assertNoText('Inheritance 4.');
+    $this->assertText('Inheritance 6.');
+    // This one is in another theme.
+    $this->assertNoText('Inheritance 5.');
+  }
+
+  /**
+   * Ensures that a module providing both templates with different engines
+   * will all be correctly displayed, no matter is the theme, using their
+   * respective engines.
+   *
+   * Tests that nyan_cat -> phptemplate template override works.
+   */
+  function testEngineCohabitationFromNyanToDefault() {
+    variable_set('theme_default', 'test_theme_nyan_cat');
+    variable_set('admin_theme', 'test_theme_nyan_cat');
+
+    $this->drupalGet('theme-test/engine-cohabitation');
+    $this->assertText('Inheritance 1.');
+    $this->assertText('Inheritance 2.');
+    $this->assertText('Inheritance 4.');
+    // This one is overriden.
+    $this->assertNoText('Inheritance 3.');
+    $this->assertText('Inheritance 5.');
+    // This one is in another theme.
+    $this->assertNoText('Inheritance 6.');
+  }
+
+  /**
+   * Ensure the subtheme has inherited of the theme engine of it's base theme.
+   */
+  function testEngineIsCorrectlyInherited() {
+    $infos = list_themes();
+
+    // Get the base theme name from the theme 'test_subtheme_nyan_cat'
+    $base_theme_name = $infos['test_subtheme_nyan_cat']->info['base theme'];
+    // Get the base theme
+    $base_theme = $infos[$base_theme_name];
+
+    // Compare their theme engine.
+    $this->assertEqual($infos['test_subtheme_nyan_cat']->info['engine'], $base_theme->info['engine']);
+  }
+
+}
diff --git a/modules/simpletest/tests/theme_test.module b/modules/simpletest/tests/theme_test.module
index 1dbc3b9564..f7dce8d8de 100644
--- a/modules/simpletest/tests/theme_test.module
+++ b/modules/simpletest/tests/theme_test.module
@@ -17,6 +17,39 @@ function theme_test_theme($existing, $type, $theme, $path) {
   $items['theme_test_foo'] = array(
     'variables' => array('foo' => NULL),
   );
+  $items['theme_test_template_test_engine'] = array(
+    'template' => 'theme_test.template_test_engine',
+    'engine' => 'nyan_cat'
+  );
+  // The following template is phptemplate per default, and will be displayed
+  // correctly even when using the 'test_theme_nyan_cat' theme, which is based
+  // upon the nyan_cat engine.
+  $items['theme_test_engine_inheritance_phpt'] = array(
+    'variables' => array(),
+    'template' => 'theme_test_engine_inheritance_phpt',
+  );
+  // The following template is nyan_cat per default, and will be displayed
+  // correctly even when using the 'test_theme' theme, which is based upon
+  // the phptemplate engine.
+  $items['theme_test_engine_inheritance_nyan'] = array(
+    'variables' => array(),
+    'template' => 'theme_test_engine_inheritance_nyan',
+    'engine' => 'nyan_cat'
+  );
+  // In opposition to the two templates above, this one is phptemplate but will
+  // be correctly overriden by the 'test_theme_nyan_cat' which has a different
+  // engine.
+  $items['theme_test_engine_inheritance_phpt_overriden'] = array(
+    'variables' => array(),
+    'template' => 'theme_test_engine_inheritance_phpt_overriden',
+  );
+  // Exact same as above, this one is nyan cat but will be correctly overriden
+  // by the 'test_theme' which has a different engine.
+  $items['theme_test_engine_inheritance_nyan_overriden'] = array(
+    'variables' => array(),
+    'template' => 'theme_test_engine_inheritance_nyan_overriden',
+    'engine' => 'nyan_cat'
+  );
   return $items;
 }
 
@@ -28,6 +61,7 @@ function theme_test_system_theme_info() {
   $themes['test_basetheme'] = drupal_get_path('module', 'theme_test') . '/themes/test_basetheme/test_basetheme.info';
   $themes['test_subtheme'] = drupal_get_path('module', 'theme_test') . '/themes/test_subtheme/test_subtheme.info';
   $themes['test_theme_nyan_cat'] = drupal_get_path('module', 'theme_test') . '/themes/test_theme_nyan_cat/test_theme_nyan_cat.info';
+  $themes['test_subtheme_nyan_cat'] = drupal_get_path('module', 'theme_test') . '/themes/test_subtheme_nyan_cat/test_subtheme_nyan_cat.info';
   return $themes;
 }
 
@@ -73,10 +107,42 @@ function theme_test_menu() {
     'access callback' => TRUE,
     'type' => MENU_CALLBACK,
   );
+  $items['theme-test/template-test-engine'] = array(
+    'page callback' => '_theme_test_template_test_engine',
+    'access callback' => TRUE,
+    'type' => MENU_CALLBACK,
+  );
+  $items['theme-test/engine-cohabitation'] = array(
+    'page callback' => '_theme_test_engine_inheritance',
+    'access callback' => TRUE,
+    'type' => MENU_CALLBACK,
+  );
+
   return $items;
 }
 
 /**
+ * Page callback, calls a theme hook suggestion.
+ */
+function _theme_test_template_test_engine() {
+  return array(
+    '#markup' => theme('theme_test_template_test_engine', array()),
+  );
+}
+
+/**
+ * Page callback, display various templates for various theme engines.
+ */
+function _theme_test_engine_inheritance() {
+  return array(
+    array('#theme' => 'theme_test_engine_inheritance_phpt'),
+    array('#theme' => 'theme_test_engine_inheritance_nyan'),
+    array('#theme' => 'theme_test_engine_inheritance_phpt_overriden'),
+    array('#theme' => 'theme_test_engine_inheritance_nyan_overriden'),
+  );
+}
+
+/**
  * Implements hook_init().
  */
 function theme_test_init() {
diff --git a/modules/simpletest/tests/theme_test.template_test_engine.nyan-cat.html b/modules/simpletest/tests/theme_test.template_test_engine.nyan-cat.html
new file mode 100644
index 0000000000..cb21cb5fd8
--- /dev/null
+++ b/modules/simpletest/tests/theme_test.template_test_engine.nyan-cat.html
@@ -0,0 +1 @@
+Success: Template overridden with Nyan Cat theme. 9kittens
diff --git a/modules/simpletest/tests/theme_test_engine_inheritance_nyan.nyan-cat.html b/modules/simpletest/tests/theme_test_engine_inheritance_nyan.nyan-cat.html
new file mode 100644
index 0000000000..6e195f981b
--- /dev/null
+++ b/modules/simpletest/tests/theme_test_engine_inheritance_nyan.nyan-cat.html
@@ -0,0 +1 @@
+Inheritance 2.
\ No newline at end of file
diff --git a/modules/simpletest/tests/theme_test_engine_inheritance_nyan_overriden.nyan-cat.html b/modules/simpletest/tests/theme_test_engine_inheritance_nyan_overriden.nyan-cat.html
new file mode 100644
index 0000000000..ba4f98b2d5
--- /dev/null
+++ b/modules/simpletest/tests/theme_test_engine_inheritance_nyan_overriden.nyan-cat.html
@@ -0,0 +1 @@
+Inheritance 4.
\ No newline at end of file
diff --git a/modules/simpletest/tests/theme_test_engine_inheritance_phpt.tpl.php b/modules/simpletest/tests/theme_test_engine_inheritance_phpt.tpl.php
new file mode 100644
index 0000000000..b6a330dfed
--- /dev/null
+++ b/modules/simpletest/tests/theme_test_engine_inheritance_phpt.tpl.php
@@ -0,0 +1 @@
+Inheritance 1.
\ No newline at end of file
diff --git a/modules/simpletest/tests/theme_test_engine_inheritance_phpt_overriden.tpl.php b/modules/simpletest/tests/theme_test_engine_inheritance_phpt_overriden.tpl.php
new file mode 100644
index 0000000000..c4ae6e5d99
--- /dev/null
+++ b/modules/simpletest/tests/theme_test_engine_inheritance_phpt_overriden.tpl.php
@@ -0,0 +1 @@
+Inheritance 3.
\ No newline at end of file
diff --git a/modules/simpletest/tests/themes/test_subtheme_nyan_cat/test_subtheme_nyan_cat.info b/modules/simpletest/tests/themes/test_subtheme_nyan_cat/test_subtheme_nyan_cat.info
new file mode 100644
index 0000000000..a656ba5671
--- /dev/null
+++ b/modules/simpletest/tests/themes/test_subtheme_nyan_cat/test_subtheme_nyan_cat.info
@@ -0,0 +1,5 @@
+name = 'Test subtheme with base theme and another theme engine'
+description = 'Test subtheme with base theme and another theme engine.'
+version = VERSION
+core = 7.x
+base theme = test_theme_nyan_cat
diff --git a/modules/simpletest/tests/themes/test_theme/templates/theme_test_engine_inheritance_nyan_overriden.tpl.php b/modules/simpletest/tests/themes/test_theme/templates/theme_test_engine_inheritance_nyan_overriden.tpl.php
new file mode 100644
index 0000000000..12bca743f6
--- /dev/null
+++ b/modules/simpletest/tests/themes/test_theme/templates/theme_test_engine_inheritance_nyan_overriden.tpl.php
@@ -0,0 +1 @@
+Inheritance 6.
\ No newline at end of file
diff --git a/modules/simpletest/tests/themes/test_theme_nyan_cat/template.theme b/modules/simpletest/tests/themes/test_theme_nyan_cat/template.theme
new file mode 100644
index 0000000000..aadbce0605
--- /dev/null
+++ b/modules/simpletest/tests/themes/test_theme_nyan_cat/template.theme
@@ -0,0 +1,13 @@
+<?php
+
+/**
+ * @file
+ * Test theme.
+ */
+
+/**
+ * Implements hook_preprocess_theme_test_template_test_engine().
+ */
+function test_theme_nyan_cat_preprocess_theme_test_template_test_engine(&$variables) {
+  $variables['kittens'] = 'All of them';
+}
\ No newline at end of file
diff --git a/modules/simpletest/tests/themes/test_theme_nyan_cat/templates/theme_test_engine_inheritance_phpt_overriden.nyan-cat.html b/modules/simpletest/tests/themes/test_theme_nyan_cat/templates/theme_test_engine_inheritance_phpt_overriden.nyan-cat.html
new file mode 100644
index 0000000000..c068ea3599
--- /dev/null
+++ b/modules/simpletest/tests/themes/test_theme_nyan_cat/templates/theme_test_engine_inheritance_phpt_overriden.nyan-cat.html
@@ -0,0 +1 @@
+Inheritance 5.
\ No newline at end of file
diff --git a/modules/system/system.module b/modules/system/system.module
index 53844d878f..4868370831 100644
--- a/modules/system/system.module
+++ b/modules/system/system.module
@@ -2533,8 +2533,12 @@ function _system_rebuild_theme_data() {
   }
 
   // Set defaults for theme info.
+  // The 'owner' key will be stored into database, 'engine' key will not and
+  // be stored into the serialized 'info' array instead. This will be loaded
+  // into system_list() and used in _drupal_theme_initialize().
   $defaults = array(
-    'engine' => 'phptemplate',
+    'engine' => NULL,
+    'owner' => NULL,
     'regions' => array(
       'sidebar_first' => 'Left sidebar',
       'sidebar_second' => 'Right sidebar',
@@ -2560,6 +2564,13 @@ function _system_rebuild_theme_data() {
     $themes[$key]->filename = $theme->uri;
     $themes[$key]->info = drupal_parse_info_file($theme->uri) + $defaults;
 
+    // Set engine on base theme only, sub-themes are allowed to inherit from
+    // their parent, so it must not be populated from this point, but will be
+    // later by traversing the theme parenting tree.
+    if (empty($themes[$key]->info['base theme']) && empty($themes[$key]->info['engine'])) {
+      $themes[$key]->info['engine'] = 'phptemplate';
+    }
+
     // The "name" key is required, but to avoid a fatal error in the menu system
     // we set a reasonable default if it is not provided.
     $themes[$key]->info += array('name' => $key);
@@ -2579,6 +2590,7 @@ function _system_rebuild_theme_data() {
     if ($themes[$key]->info['engine'] == 'theme') {
       $filename = dirname($themes[$key]->uri) . '/' . $themes[$key]->name . '.theme';
       if (file_exists($filename)) {
+        $themes[$key]->engine = 'theme';
         $themes[$key]->owner = $filename;
         $themes[$key]->prefix = $key;
       }
@@ -2586,6 +2598,7 @@ function _system_rebuild_theme_data() {
     else {
       $engine = $themes[$key]->info['engine'];
       if (isset($engines[$engine])) {
+        $themes[$key]->engine = $engine;
         $themes[$key]->owner = $engines[$engine]->uri;
         $themes[$key]->prefix = $engines[$engine]->name;
         $themes[$key]->template = TRUE;
@@ -2616,16 +2629,17 @@ function _system_rebuild_theme_data() {
       $themes[$base_theme]->sub_themes[$key] = $themes[$key]->info['name'];
     }
     // Copy the 'owner' and 'engine' over if the top level theme uses a theme
-    // engine.
-    if (isset($themes[$base_key]->owner)) {
-      if (isset($themes[$base_key]->info['engine'])) {
-        $themes[$key]->info['engine'] = $themes[$base_key]->info['engine'];
-        $themes[$key]->owner = $themes[$base_key]->owner;
-        $themes[$key]->prefix = $themes[$base_key]->prefix;
-      }
-      else {
-        $themes[$key]->prefix = $key;
-      }
+    // engine and the current theme has no engine itself.
+    // Note that 'engine' must be stored into the info array because it has no
+    // associated column in the {system} table whereas the 'owner' column
+    // exists.
+    if (isset($themes[$base_key]->engine) && !isset($themes[$key]->engine)) {
+      $themes[$key]->engine = $themes[$key]->info['engine'] = $themes[$base_key]->engine;
+      $themes[$key]->owner = $themes[$base_key]->owner;
+      $themes[$key]->prefix = $themes[$base_key]->prefix;
+    }
+    else {
+      $themes[$key]->prefix = $key;
     }
   }